The interactions of the vascular endothelial surface with blood cells, with the coagulation system, with hormones and other modulatory molecules and with the immune system are determined by the various membrane proteins of the endothelial cell surface. The proposed research will examine the surface membrane protein composition, distributions and lateral mobilities in vascular endothelial cells. The effect of thrombin, a modulator of endothelial metabolism on these parameters, will also be examined. The human endothelial cell thrombin receptor and human endothelial cell-specific alloantigens, endothelial membrane proteins of clinical importance, will be identified, characterized and purified. Methods will be developed for plasma membrane subractionation and "supravectorial" (i.e., superior surface only) labeling so as to identify selectively the membrane proteins of the endothelial luminal surface. Monoclonal antibodies will be raised against human endothelial cell surface antigens, both for identification and for affinity purification of endothelial membrane proteins. These antibodies will also be used as fluorescent labels for fluorescence microscopy and fluorescence photobleach recovery experiments of endothelial surface proteins. The long-range objective of this work is to understand the function of both the normal and pathologically-altered endothelial vascular lining in terms of macromolecular structure.